The Mayo Clinic says Bipolar II disorder is a treatable condition, affecting parts of the brain that control drive, thought and emotion. This type of depression is most likely caused by a set of genetic and environmental factors.

Klein said the 47-year-old congressman underwent gastric bypass surgery back in 2004, which changes how the body absorbs things like foods, liquids and medications.

There's no timeline for Jackson's recovery, according to Jackson's father, civil rights activist Rev. Jesse Jackson. The elder Jackson said earlier this month that his son has no plans to step down from his post.

Jackson is currently under investigation by the House Ethics Committee for his connections to former Illinois Gov. Rod Blagojevich, who was convicted on corruption charges for trying to sell off Barack Obama's Senate seat after Obama was elected president. Jackson has repeatedly denied wrongdoing in the scandal and has not been charged.

Former Rhode Island Rep. Patrick Kennedy said Monday he plans to visit Jackson on Thursday at the Mayo Clinic, The Associated Press reported. Kennedy was treated at the Mayo Clinic himself for addiction and depression in 2006 after a late-night car crash at the U.S. Capitol. He decided not to run for a ninth term in 2010 and retired from Congress last year.